Small nudibranch difficult to see clearly even with hand lens - gills, rhinophores & shape suggestive of Onchiodorid - but I'm a beginner.
No interactions present.
(and no Ispots in CoL or SANBI either)
I'd thought the rhinophores were retracted, but pictures of it show they're well camouflaged, as are the gills
http://www.conchsoc.org/spAccount/goniodoris-castanea .The lateral view shows the prominent rim just before the rhinophores,as does your photo
I am greatly encouraged by your comments, so I'll post another nudibranch.
Lat/Lng: 51.211, -4.1174
OS grid ref: SS521478
The beach lies seaward of the terrace across car park with sea defences, marine location in urban environment